I am 18 years old and speeding naked through the Colorado desert in an open-top BMW. It's night time and it's cold but the heated leather seats are keeping me warm. In the driver's seat is a handsome fortysomething man whom I first encountered on the internet. We're heading for Vegas. He's a car salesman, novelist and Mormon. I'm a student enjoying a rather exhibitionist moment during a fairly extraordinary Easter break. But how did I get here? What am I doing? And what will my mother think?

Doubtless Mark Oaten asked himself the same questions when his private life suddenly became very public. Bewitched, bothered and bewildered in the face of intense media scrutiny, he admitted having sex with men that he'd found online. The website? Gaydar.co.uk. Oaten is far from alone.

Gaydar has 3.5 million members globally. The UK is by far its biggest market, with a total of almost one million members. Most are in big towns and cities, especially London, Manchester and Brighton. Quite a few live near you. Far from being dirty old men, most members are between 25 and 34.

At any time of the day or night, wherever you are in the world, these men are online arranging to get it on. There are leather queens in LA, sugar daddies in Shanghai, and, at every point of the compass, married men seeking "discreet meets". Welcome to the world of Gaydar, where it's every man for himself.

When Freud talked about "polymorphous perversity" he foresaw Gaydar. The site caters for any and every preference. You name it (or even if you can't), it's there. Specialised chatrooms include chastity, chubbychasers, Eurovision, hypnosis, kilts and scallies. There is plenty to amuse and arouse and much to disgust, but absolutely nothing illegal.

A powerful feature called GPS - Gaydar positioning system - allows gay men to find like-minded souls wherever they are in the world. If they're travelling, they can enter an itinerary so that they can meet locals and line up dates, trips or whatever. No more "is he, isn't he?" No more wandering around a foreign town hoping to find the single and doubtless dodgy gay bar.

If they want to send more than just a few messages and look at more than a handful of profiles each day, they have to pay. Communication costs, and Gaydar membership is £60 a year.

Given all these features, it's easy to see why Gaydar claims to be the "ultimate gay personals website". And according to HitWise, the internet traffic analysts, it's true. In December 2005, Gaydar accounted for more than half of all gay website visits.

So far, so gay.

The real surprise is that Gaydar is much more popular than straight competitors such as Match.com. According to HitWise, it is consistently the number one lifestyle/dating website, despite having a much smaller target audience.

What's even more surprising is that a website aimed at a sexual minority is now more popular than the websites for Marks & Spencer, National Rail and Ryanair. Statistically, online, you are now more likely to hook up with a stranger than book a trip or buy a sofa. Gaydar is no longer just a website. It's a cultural phenomenon and a community.

I have a Gaydar profile. As does almost every gay man I know. A profile is increasingly a vital component of being gay. However you use it, you just have to have it. It's a badge of identity, an essential accessory, a passport to a big gay world. And where gay culture leads, straight culture follows.

Registering a profile is easy and free. The profile needs to stand out - a subscriber must sell himself (even if they're not one of the thousands of "escorts" with a more "commercial" profile). Gaydar is a heady combination of objectification, commodification and fantasy. Now give your details. These include: age, job, body type, body hair, penis size, sexual role, fetishes and whether you smoke, drink or take drugs. You can include hobbies. There's a Tardis-load of gay Dr Who fans. Dame Judi Dench is, it would seem, everyone's "favourite actress". Brokeback Mountain is currently everyone's "favourite movie".

Men don't have to post a photograph, but an air of mystery will only get them so far. Putting up a picture of a celebrity and writing "Not me but looks like me" is a popular - but pilloried - option. What have you got to hide?

Thrillingly, subscribers can see who has viewed their profile and what they think. A star means "nice". A flame means "hot". Nothing means they think nothing of you. Just as I check my bank balance online and my books sales on Amazon, I like to look and see who's looking at me. Mirror, mirror, on the web...

The UK is the largest online dating market in Europe - last year we spent £31m looking for love online. "That figure will almost double by 2008," predicts Nate Elliott, an online-dating analyst at JupiterResearch. Once the preserve of geeks and freaks, it seems that online dating is now everyday and OK. "A few years ago you could have claimed that strongly religious or elderly individuals were generally against online dating. But now there are sites like christiansingles and primesingles."

Whoever you are, whatever you're into, there is a site out there for you. Others include vegansingles, bikerdate and lovehorse.

"There will always be those who don't like the idea of online dating," continues Elliott. "That stigma will never disappear. But the people you see online now are more like the people you see in everyday life." Except that the people I see in the street don't usually have their bits out.

Love is big business. Lust is bigger. This year, Europeans will spend £157m on online dating. They will spend £291m on "adult content" - almost twice as much. Go figure. Sex is what's really getting us online in such record numbers.

"Most of the big dating sites claim to be about actual dating aiming at actual relationships," says Elliott. "Sites like Match.com and Datingdirect say they want people to meet and fall in love. Users say that friendship is their main goal, not physical intimacy."

Liars.

The current advertising campaign for Match.com is the UK's biggest online dating promotion. In slightly tittersome language, it details the "common side-effects of being in love". Frustratingly, it makes no mention of sex. The website itself is self-consciously nonsexual - it's all neutered pastel colours and love hearts. It's very girly. Its slogan? "We guarantee you'll find someone special within six months."

Gaydar's homepage features pictures of semi-naked men beckoning you to log on. Its slogan? "What you want, when you want it."

Which is more honest? And which approach do you prefer - honestly?

"People definitely lie about what they're looking for," says Dr Monica Whitty, social psychologist at Queen's University and co-author of Cyberspace Romance: the Psychology of Online Dating.

And despite increasing evidence that in the post-feminist world of ladettes and speed-dating there is a convergence in the sexual behaviour of men and women, Whitty says that the traditional gender roles still apply online.

"It's still not socially acceptable for women to seek casual sex," she says. "Often women will try to attract more men by claiming they're only after a casual relationship. Men will do the opposite. Both sexes are playing a game."

On Gaydar, the game is different because there are no female players. It's a gay man's world.

"Gaydar is mostly about sex," says Tim Fountain, who wrote the play Sex Addict about his Gaydar experiences. Fountain claims to have had sex with more than 5,500 men in his 30-odd years. "A lot from Gaydar. I've also made friends. I am not saying you can't find a relationship there, but it's a bit like going to a sauna or backroom and expecting to see a wedding."

Whitty's research on straight sites shows that women tend to tinker with their vital statistics; they exaggerate the size of their breasts and shrink their waist measurements. Men usually inflate their socioeconomic status. "Outright lies are told, but more often than not it's simply exaggeration." For gay men, the pay packet is as important as the package. Often, both are exaggerated.

"A lot of gay men pretend to be married," says Fountain, "because married men are such a turn-on. If I change my profile to 'bisexual' I get more hits."

"Because there are so many choices you need to attract attention," says Whitty. "You need to be attractive and appealing but not disappointing when you meet in the flesh."

Fountain's Gaydar profile is Steeee77. Having met him, I can say it's fairly accurate. He's seeking "sex and more with right person". I was surprised to learn that he likes "denim and nipples". But hey ho. "The more like you your profile is, the more likely an offline relationship is to succeed," says Dr Whitty. "Technology, especially digital cameras, makes it easy to cheat - but if you meet, you will get caught."

On Gaydar it's all subjective, and therefore potentially deceptive. Gaydar years, for example, are more akin to dog years. I've skipped a birthday or two. But if you are prepared to fib about your body shape or age, how else will you bend the truth?

During my moment of teenage madness in the US I discovered that the man I had flown hundreds of miles to meet was: 10 years older than he had claimed, not just a Mormon but a bishop, and his novels were self-published. Even the BMW was borrowed.

The men behind Gaydar have never granted a media interview before. It's not that surprising, considering they have been blamed for, among other things, the rise in sexually transmitted infections.

Gary Frisch, 37, and Henry Badenhorst, 37, met off-line in their native South Africa and moved here in 1997. They have an IT background and live "outside London". They arrive for our interview clad in business suits and don't even have Gaydar profiles. It's all very sensible. Nothing like the product which now makes them around £1million a year in the UK alone.

"Gaydar is our most successful brand," says Badenhorst. Their company, Q Soft Consulting, currently employs a total of 40 people and owns, among other products, Gaydarradio and Gaydartravel. They plan to extend into other "lifestyle" areas, and already sponsor Sydney Mardi Gras.

"We created Gaydar in 1999 for a friend who had trouble finding a partner," says Frisch, smiling. "So far he's found quite a few."

Today Gaydar has more than three million members, while Match.com has just 500,000 gay members.

Despite being banned from the site because his play could potentially have involved outing Gaydar members, Tim Fountain remains a devout fan of the site. "If you're young and gay, you're no longer forced like I was to hang out in public lavs hoping to meet people like you."

Peter Tatchell, the gay human rights activist, agrees: "The internet has been a great boon for isolated individuals and it's made meeting people easier, faster and more convenient." But he has never gone on Gaydar - he prefers good old-fashioned cruising.

"Whatever the downside to bars and parks," he continues. "I like seeing the person and interacting face-to-face. I am also put off by horror stories where people arrange a rendezvous and the partner has turned out to be nothing like his description. With Gaydar, more and more encounters have the character of wham, bam, thank you man. To some degree it has coarsened gay life."

Is a gay bar really less coarse than a gay chatroom?

"I supposed both can be sociable or unsociable," admits Tatchell.

Gaydar has changed the way gay men socialise. Now, we log on and only go out once a meeting - with friend, partners or whatever - has been arranged. No more standing around. There is some anecdotal evidence to suggest that gay venues are feeling the pinch. An article in The Economist earlier this month reported that gay businesses in the Canal Street area of Manchester are experiencing a downturn which they attribute in part to the success of Gaydar.

"That's rubbish," counters Frisch. "Bad gay venues are just looking to assign blame for their own commercial failures. Of course men still go out to meet other men."

But Gaydar does allow for a different - and sometimes unhealthy - approach to meeting other men. "It's good that profiles allow people to be specific about what they want," explains Tatchell. "But the anonymity emboldens some to express racist and other objectionable opinions they wouldn't dare utter in a bar or café."

This is true. Countless profiles say: "No fats or femmes". Phrases like "I am not racist but I don't do Asians or blacks" are sadly commonplace. As is ageism. Body fascism is pretty much standard.

"People are entitled to their preferences," says Frisch. "It's not for us to judge - we leave that to the users. There are lots of discussions - often very intense - but we stand back." It takes a lot to get banned from Gaydar, as Fountain found out. The website Sexualracismsux.com is where racist Gaydar members are named and shamed.

Unsafe sex is the most worrying predilection. Barebacking - anal sex without a condom - is one of the few practices not allotted a Gaydar chatroom. Yet a cursory search of the site reveals more than 600 members who admit to being into barebacking. It's Bareback Mountain.

"I feel very uncomfortable with that," says Tatchell. "I don't want to police anyone's behaviour but sexual rights entail sexual responsibilities. Barebacking risks creating a new untreatable superstrain of HIV."

"We can't stop people from barebacking," argues Badenhorst. "And banning users would only drive it underground. This way, others know who is into what." This month Gaydar launched Netreach, a sexual health chatroom run by the Terrence Higgins Trust. "I think that's a great idea," says Fountain. "Especially for slappers."

So Gaydar is just another space like a café, bar or club. The people in it decide how to behave. But is it a private or public space?

"It's private," says Frisch. "We have lots of celebrities on our site and only one or two, like Boy George, are out. We want them to feel as safe as regular people." Even if a politician was practising online the opposite of what he preached offline? "Even then."

"I have no problem with public figures having Gaydar profiles even if they're closeted," says Tatchell. "They have a right to a sex life and should be left alone unless they're being hypocritical."

But what about decency in public life? Do we need to know how Chris Bryant, Labour MP for Rhondda, looks in tight white underpants? As "Alfa101", he advertised himself as "very versatile". Isn't versatility a valued quality in politics? Not according to the tabloid press - Bryant's reputation was trashed. But he remains an MP. And so he should.

"Everyone who goes on Gaydar knows what to expect," says Tatchell. "Tabloid journalists have no more right to intrude there than they have to peep through net curtains."

Last month a local councillor in County Wexford, Ireland, defended his right to a Gaydar profile, and likened it to a personal ad in a newspaper. Malcolm Byrne's profile included non-explicit photos and harmless details about hobbies. In an interview with the Irish Independent, Byrne, 31, said: "I happen to be a public representative who is gay. I have nothing to be ashamed of."

Fountain, on the other hand, thinks politicians need to get real. "I don't know how Oaten thought he could remain anonymous. Gaydar is a public space which we access in the privacy of our own homes. I wouldn't put anything up there I didn't want everyone to know."

As far as Dr Whitty is concerned, Gaydar is a vision of the future. One she's quite looking forward to.

"Gaydar facilitates existing sexual desires - it doesn't create them," she says. "Straight men and women have similar desires, but so far they've not been allowed to act on them. Gay culture has led the way."

Women are already catching on. Gaydargirls.com launches next month. Already 140,000 gay women have signed up, confounding the prevailing myth that lesbians prefer deep-and-meaningful relationships to down-and-dirty sex. Another fight won, another step forward for sisters everywhere.

"I had those stereotypes too," says Badenhorst . "But it seems that young metropolitan lesbians are behaving more and more like gay men. They've just taken longer to get to the same place."

So Gaydar is blazing a trail which straight culture is already starting to follow. Like it or hate it, this is increasingly a world where everything is available at the tap of a keypad and the click of a mouse. CDs, groceries... and now, whatever your orientation, sex. I happen to think it's great. But Mum, if you're reading this, I'd still rather you didn't go on Gaydar.